PROJECT SUMMARY Physiologic, molecular, and genetic observations all point to impaired intestinal epithelial function as a key element in the multifactorial pathogenesis of inflammatory bowel disease (IBD). The lack of treatments directed at promoting epithelial homeostasis represents a conspicuous weakness of our current IBD therapeutic armamentarium. Therefore, research is needed that will advance our understanding of mechanisms to preserve epithelial homeostasis in the setting of inflammation. IL-33 is a member of the IL-1 cytokine family that signals through the IL-1 receptor related protein ST2. IL-33 is markedly upregulated in the intestinal mucosa of patients with UC and CD, and single nucleotide polymorphisms (SNPs) in a linkage disequilibrium block containing the gene for ST2 (IL1RL1) are associated with risk for IBD. Genetic deletion of either IL-33 or ST2 results in exacerbation of murine colitis, suggesting a protective effect for IL-33 signaling. While colon epithelial cells express ST2, little is known regarding the direct effects of IL-33 on colon epithelium. Our preliminary data support the hypothesis that IL-33-ST2 signaling in the intestinal epithelium induces goblet cell differentiation and augments barrier function, which is impeded by IBD-associated SNPs within the IL1RL1 locus. In Aim1, we will use primary murine enteroids and two-dimensional monolayers derived from wild type (WT) and ST2?/? mice to determine the direct effects of IL-33-ST2 signaling on colon epithelial cell differentiation and barrier function. In Aim 2, we will use primary colonoid cultures derived from genotyped pediatric IBD and non-IBD patients to determine the effects of IBD-associated SNPs within the IL1RL1 locus on human colon epithelium. This research will elucidate how cytokines preserve or augment epithelial barrier functions in the setting of colitis, and how IBD risk genes impair this response, which may uncover novel therapeutic strategies to preserve and restore epithelial homeostasis in IBD.